Soil gas analysis



\Aug. 27, 1940.

T. H. DUNN I son. GAS ANALYSIS: Filed July 26,1938

2 Shets-Sheet l INVENTOR Thomas, H. Dunn ATTORNEY -'Au .27,.194o. J H UNN. 2,212,681

son. GAS ANALYSIS Filed July 25, less 2 Sheets-Sha n? 70 Soil Gas I Analysis Apparatus INVENTOR J Thomas HIDunn ATTORNEY Patented Aug. 27 1940 UNITED STATES PATENT QFFlCE SOIL GAS ANALYSIS Thomas H. Dunn, Tulsa, Okla", assignor to Stanolind Oil and Gas Company, Tulsa, Okla a corporation of Delaware Application July 26, 1938, Serial No. 221,334

Claims.

' present in minute traces in surface soils immediwhat horizontally offset from, the deep petrole-' ately above, or more generally above and someum-bearing or natural gas-bearing deposits. In many cases the hydrocarbon soil gases are not found in substantial quantities immediately above the deposit but rather in an area surrounding the deposit. .In other cases the gases appear to seep or diffuse upward inminute amounts over geological periods of time along fault planes and the like. In any event, the analysis of these soil hydrocarbons present in quantities measured in parts per million or parts per billion has proven to be a valuable method of geochemical prospecting for deep seated hydrocarbon reservoirs.

Since methane is often associated with coal deposits and is commonly formed by the decay of organic matter in the soil, the methane content or total hydrocarbon content alone is not a reliable guide in this type of geophysical prospect- .ing work and it becomes of the highestimportance to determine with great accuracy theamount of ethane and heavier hydrocarbon gases present in the soil gases and it is to this phase of the problem that my invention is principally directed.

Two alternative methods of obtaining the soil gas sample are available. The first is to pump a sample of the soil gas from the soil, preferably. at a depth of from a foot or two to as much as several hundred feet, under carefully standardized conditions, and then take this gas sample to the laboratory for analysis as will hereinafter be described. This sampling operation can be done, for example, by the use of the apparatus described in J. BA Clark's U. S. Patent No.

v advantages of my inventionwill'become apparent ject of my invention is to provide new and superior methods and apparatus for the analysis of soil gases todetermine the amount of ethane and heavier hydrocarbon gases present. It is also an object of my invention to provide novel and 5 improved methods and apparatus for the quantitative separation of methane from ethane and heavier hydrocarbons in micro gas analysis. Further, and more detailed objects, uses, and

as the description thereof proceeds.

The invention will now be'described with particular reference to the accompanying drawings which form a part 'of this specification, and are -to be read in conjunction therewith. 15

In the drawings, Figure 1 is,a diagrammatic elevation of my apparatus as used in the analysis of gas samples withdrawn directly from the soil in situ; and Figure 2 is a diagrammatic elevation of auxiliary apparatus :used to remove and purifygas samples from soil sampled as such.

- Turning'now to Figure 1, the soil gas sample container II is connectedto' sample inlet line l2 and valve i3 is opened. Valve I4 is also opened and stopcock I5 is opened to the atmosphere. 88 Water is then forced into container 1 l by raising leveling bottle l6, thus displacing gas from the sample container, which passes through potassium hydroxide solution in tube I1 and successive layers of phosphorous pentoxide, Ascarite, and phosphorous pentoxide in tube l8, and thence out through stopcock I5. The purpose of this operation is to flush this portion of the apparatus.

At this point, or at a previous stage in the procedure, the apparatus is evacuated by means of two s'tage'mercury diffusion pump l9 and "Hy- Vac-pump 20, the various valves to the left of stopcock l5 being open. The vacuum pumps are preceded by trap 2i immersed in bath- 22, which may contain acetone and carbon dioxide, or other refrigerating medium, to liquefy any condensable material and reduce the load on .the vacuum pumps.

Cut off valve 23 is then closed while similar valves 24 to 21 remain open. These valves are mercury reservoirs connected by three-way stopcocks 28 alternatively to suction line 29 or to at-. mosphere. Suction is maintained on line 29, for

instance by means of water aspirator-30. When suction is applied to these valves the mercury levels are lowered to points below the bottoms of We 3| to 35, and the valves are then open. while when the reservoirs are vented to the atmosphere the mercury rises into the U's and the valves are closed.

I Atfthisptage stopcock l is turned to connect tube-1'8"wvith chamber 36, and mercury reservoir the ethane and heavier hydrocarbons.

31 is lowered to bring the mercury level below the bottom of the tube in chamber 36, and a portion of the sample, for instance 200 c. c., is allowed to enter the main part of the apparatus.

McLeod gage 38 is filled with mercury by screwing down plunger 39, and, after the samplehas' been admitted, cut oil valve 21 is closed while out ofi valve 23 remains closed and cut off valves 24 and ,25 remain closed. The mercury in valve 26 is allowed to rise a short distance above the bottom of U 33, to act as a check valve for Toepler pump 40. This pump is mercury filled and is operated, just as are the out off valves, by manipulating three-way stopcock 41'. 'When this pump operates .the mercury rises into chamber Y42 and the lower portions of tubes 43 and 44. The operation of this pump forces the sample back and forth over phosphorous pentoxide in tube 46 and serves to remove the last traces of water. I find it desirable to continue the pumping for about 30 minutes.

Next a flask 41 containing liquid air is placed around condensing chamber 48, which is connected by tube 49 with U 3!, and by tube 50 with U 32. Pumping is then continued for another 30 minutes by meansof Toepler pump 40, thus circulating the gas through condensing chamber 48 to condense at least part of the methane and all of the ethane and heavier hydrocarbons. The gas enters chamber 48 through tube 69, and leaves through. tube 50. The mercury is then circulated in the McLeod gage several times to insure condensation of the hydrocarbons contained in the small amount of gas in the dead end zone 5!, between the condensing chamber and the McLeod gage. v

' In this manner the total hydrocarbon content of the sample introduced into the apparatus is condensed in chamber 48.

The next step is to remove the methane from It is most important that this be done completely and quantitatively, and the-method of doing this constitutes one of the most important features 01 my invention. In short, I have found that this can be accomplished by the application of a "very high vacuum such as to create pressures of less and preferably less than about 0.05 micron. In

than about 0.1 micron (0.0001 cm.oi mercury),

fact, in actual practice I use pressures of from 0.02 to 0.04 micron. 1

This is done by opening .-.cut off valve 23'to remove mercury from U 34 and starting vacuum pumps i9 and 20. The pumping is continued for approximately two hours, until a pressure of the order of magnitude mentioned is reached. .The methane is thus pumped ofi and discarded, while the ethane remains inqcondensing chamber 48 by virtue of the low temperature of the liquid air bath. I have found that I can thus remove the methane completely without vaporizing any substantial amount of the ethane.

Cut oi! valves 3| and 32 are then closed so that the condensing chamber and the McLeod gage are isolated from the remainder of the .system. This isolated portion of the system has previously been calibrated by the use of known synthetic" samples, so that a given pressure rise as measured on McLeod gage 38 can be converted to a known volume of heavy hydrocarbons under standard conditions. The flask of liquid air 41 is removed from the condensing chamber 48, and the condensing chamber'is allowed to come to room temperature, while the pressure of the system is measured by the McLeod gage. When a constant reading is obtained the volume of the ethane and heavier hydrocarbons is calculated directly from the pressure rise. 6

While liquid air is convenient for use in flask 41, other liquefied gases, for instance oxygen, or preferably nitrogen, which give approximately liquid air temperatures, can be used.

For many purposes the volume of ethane and 0 heavier hydrocarbons, determined as above outlined, is sutlicient, but I prefer to go further and determine the amount of carbon dioxide formed by burning these hydrocarbons.

To do this, approximately 200 c. c. of air, preiii viously purified by slowly aspirating it through a tube'filled with activated charcoal immersed in liquid air, to remove carbon dioxide and hydrocarbon gases, is admitted into the apparatus through out 01f valve 21, as was the original sample. The air plus the ethane and heavier hydrocarbons is circulated through drying tube 46 for about 30 minutes, by the use of Toepler pump 40, exactly as was the original sample. The air-gas mixture is next circulated for 30 minutes over combustion tube 52, containing an electrically-heated platinum coil 53. This burns the hydrocarbons and the water formed is removedin the same operation by virtue of drying tube 56.

Condensing chamber 58 is then again immersed in liquid air, or 0 her low temperature bath, to condense the canon dioxide formed as a result of the combustion of the hydrocarbons. The system is then again evacuated to a'pres- 35 sure of about 0.02 to 0.04 micron, valves 24 and 25 are closed, bath 41 is removed, the carbon dioxideis allowed to vaporize, and the pressure rise is measured exactly as was done in the direct measurement of the amount of ethane and heavier hydrocarbons.

This carbon dioxide determination gives a quantitative value for the amount of ethane and heavier hydrocarbons in the soil gas sample, and the ratio of the volume of these hydrocarbons to the volume of carbon dioxide formed from them of methane in a soil gas sample, as well as the amount of ethane and heavier hydrocarbons, I next proceed to determine the carbon dioxide formed by burning a sample of the total soil gas hydrocarbons. From this, the amount of carbon dioxide formed from the methane can be determined and the volume of this carbon dioxide is, of course, equal to the volume of the methane from which it was formed.

To make this determination, the system is once 00 more evacuated to a very low pressure, another sample of soil gas is introduced as before, the last traces of water are removed, the hydrocarbons are'condensed, their pressure rise on warming isobserved, purified air is introduced, the 65 hydrocarbons are burned, and the carbon dioxide is measure all in accordance with the technique previously outlined.

My method and apparatus can be applied equally well in connection with soil gas surveys thereof, it is to be understood that these are by 1, is of carefully sealed all glass construction,

to eliminate any leakage under high vacuum.

calcium chloride drying tube I05, and thence through phosphorus pentoxide and Ascarite U-tubes I08- and I01, to remove water and carbon dioxide respectively. It then passes through i U I08, which can be shut on by valve I09, which operates in the same manner as valves 23 to 2] of Figure 1. The sample then enters the evacuated analytical system at point IIO.

A liquid, such as glycerine, which will be selectively sorbed by the soil and replace the hydrocarbons, can be added to the soil in the retort.

To conduct the test,'va1ves 23*, 24, and 25 (Figure 1), are opened, as is valve I09 (Figure 2).

By operating "Hy-Vac pump 20,'the retort can be evacuated to about 0.1 cm. of mercury absolute pressure. The temperature of the retort is then gradually raised to between 600 F., and 800 F., depending on the nature of the sample. The condensing chamber 48 is next cooled with liquid air to condense the hydrocarbon vapors given off in the retorting operation. The retort 1s maintained at the prescribed temperature for one hour. The amount of ethane and heavier hydrocarbons present can then be determined by the method previously described.

While I have described my invention in connection with certain preferred embodiments way of example andnot by way of limitation, and

1 do not mean to be restricted thereby, but only to the scope of the appended claims.

I claim:

I 1. In a method of soil gas analysis for minute traces of hydrocarbons heavier than methane present together with methane in said soil gas, the steps which comprise condensing at least a substantial part of said methane together with substantially all of said heavier hydrocarbons densed hydrocarbons in a single operation while at approximately liquid air temperature, without removing any substantial amount of said heavier hydrocarbons, by maintaining the pressure on said condensed hydrocarbons at a value less than about 0.1 micron for a substantial period of time, and determining the amount of the residual heavier. hydrocarbons.

2. In a method of soil gas analysis for minute traces of hydrocarbons heavier than methane present together with methane in said soil gas, the steps which comprise condensing at least a substantialpart of said methane together with substantially all of said heavier hydrocarbons at approximately liquid-air temperature, quantitatively removing said methane from said, condensed hydrocarbons in a single operation while at approximately liquid air temperature, without removing any substantial amount oi. said heavier hydrocarbons, by maintaining the pressure on said condensed hydrocarbons at a valueless than about 0.1 micron tor a substantial vaporized hydrocarbons under standardized conditions.

carbons, and measuring the pressure said 3. In a method of soil gas analysis for minute. I

traces of ethane and other hydrocanbons heavier than methane present together with methane in said soil gas, the steps which comprise condensing at least a substantial part of said methane together with substantially all oisaid heavier hydrocarbons at approximately liquid air temperature, quantitatively removing said methane from said condensed hydrocarbons in a single operation while at approximately liquid air temperature, without removing any substantial amount of said heavier hydrocarbons, by main taining the pressure on said condensed hydrocarbons at a value less than about 0.05 micron for a substantial period of time, vaporizing the residual condensed hydrocarbons,- and determining the amount of heavier hydrocarbons by observation of the vapor pressure in the system.

4. In a method 01 soil gas analysis for minute traces of ethane and heavier hydrocarbons present together with methane in said soil gas, the

steps which comprise condensing at least av substantial part oi said methane together with substantially all of said heavier hydrocarbons at approximately liquid air temperature, quantitatively removing said methane from said con densed hydrocarbons in a single operation while at approximately liquid air temperature, without removing any substantial 'amount of said heavier hydrocarbons, by maintaining the pressure on said condensed hydrocarbons at a value less than about 0.1 micron tor a substantial period of time, burning the residual hydrocarbons, removing water trom the system, condensing the carbon dioxide formed, removing fixed gases by application of a-high vacuum, vaporizing the carbon dioxide, and measuring the pressure of the carbon dioxide vapors as an index of i the amount or said ethane and heavier hydrocarbons present in the sample.

5. In a method of soil gas analysis for minute 1 traces of hydrocarbons heavier than methane present togetherwith methane in said soil gas, the steps which comprise condensing at least a substantial part of said methane together with substantially all of said heavier hydrocarbons at approximately liquid air temperature, quantitatively removing said methane from said condensed hydrocarbons in asingle operation while proximatecomposition of said residual hydro-,

carbons, burning said residual hydrocarbons, measuring the amount of carbon dioxide thusiormed to determine the, approximate composition of said residual hydrocarbons, burning a sample or the total hydrocarbons in said soil gas sample, and measuring the amount of carbon dioxide thus formedto permit calculation of the amount of methane in said soil gas sample.

' THOMAS E. DUNN. 

